The invention relates to an improved method and apparatus for the common grinding of two or more brittle materials having different grinding properties, such as portland cement, which is a more easily grindable material and blast furnace slag, which is a more difficult to grind material.
In the calcining of portland cement clinker from cement raw meal, the technically possible specific heat consumption as stabilized in recent years, is approximately 750 kcal/kg clinkers. For further energy savings, a greater opportunity is possible in the manufacture of mixed cements. That is, in the admixture of such materials to portland cement which do not need to be treated with the expenditure of heat. Such a material is granulated blast furnace slag for the manufacture of slag portland cement or blast furnace cement. With the use of this material, energy savings result from the substitution of from 10% to 75% of the cement clinker which is manufactured with a high energy expenditure with granulated blast furnace slag having latent hydraulic porperties. For exmaple, in current European community practice, cement utilizing approximately 70% portland cement and approximately 22% slag cement is produced. According to the German industrial standard (DIN), both components, namely portland cement clinker and granulated blast furnace slag must be commonly ground together. This is customarily done in a rotating cylindrical ball mill.
Common grinding of the slag with the portland cement clinker creates problems insofar as the two components have different grinding properties. The portland cement clinker is easier to grind, and the slag is more difficult to grind. Efforts have been made heretofore to attempt to grind the materials together, but this has not been successful, and one attempt has included separately grinding the products in different mills and thereafter mixing the ground products. In other manufacturing processes, the problem is presented of simultaneously grinding different products other than portland cement clinker and blast furnace slag wherein the materials have different grinding properties and must be ground together.
It is accordingly an object of the present invention to provide an improved method and apparatus which is capable of accomplishing the common grinding of materials having different grinding properties and wherein despite the fact of these different grinding properties, the resultant product has an equal or virtually equal granular size distribution.
A further object of the invention is to provide an improved method and apparatus for common grinding of materials having different grinding properties wherein the steps of grinding make possible the use of compact unitary apparatus and wherein the process and apparatus assures that the finished product is uniform and undesirable materials of incorrect particle size are not included in the finished product.
In accordance with a feature of the invention, there is involved a pretreatment of the material which is more difficult to grind, such as the slag, and this pretreatment includes improvement of the characteristics of the slag so that it is subsequently capable of being ground together with the material which is easier to grind, such as the portland cement clinker. After the pretreatment of the material, the harder to grind and the easier to grind substances are admixed and delivered to a common grinding apparatus. By the improvement of the grinding capability of the material which is more difficult to grind, virtually equal granular size distribution of all materials is attained so that uniform common grinding can be accomplished. This has as a consequence the fact that the solidification of the slag portland cement or blast furnace cement, which has heretofore been disadvantageously slow is eliminated, or substantially improved and matched to the solidification of the portland cement.
The pretreatment of the material which is more difficult to grind, prior to its common grinding with material which is more easy to grind, is accomplished by means of a pressure crushing of the first more difficult to grind material, preferably in the nip of rollers of a roller machine at a high pressure force. A roller grinder wherein the rollers are operated at a grinding pressure of more than 2 t/cm roller length is utilized. The roller grinder for the purposes of pressure crushing thus operates in accordance with the needs of the harder to grind material and operates at pressures in accordance with principles of individual granular crushing or product bed crushing or a combination of said individual granular crushing or product bed crushing. With the high pressure forces, product agglomerates can be formed from the precrushed materials of the substance which is more difficult to grind. The product agglomerates are then supplied and admixed with the substance which is easier to grind, and the two are supplied to a mill wherein the formed agglomerates are broken up and all substances are ground to uniform end fineness. The granular size of the significant portion of the material which is more difficult to grind can be greater or smaller than the width of the nip of the rollers.
In accordance with the present concepts, the pressure force of the roller grinder is so high that as great as possible a destruction of the particles of the material which is more difficult to grind is achieved. By product bed crushing referred to hereinabove, is meant that the more difficult to grind charging product is supplied to the nip of the rollers in such a large quantity that the product to be crushed is drawn between the rollers forces the rollers apart, and the particles of the charging product mutually crush one another in the nip of the rollers. Thus, in one charging a product bed pulverization is accomplished. A nip width between the rollers which are forced against one another in a resilient fashion thus develops which is greater than the particle size of the product fed therebetween.
Other objects, advantages and features will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiment thereof in the specification, claims, and drawings, in which: